Leather-like sheet material principally composed of ultra-fine fibers and polyurethane has outstanding characteristics not found in natural leather and can be widely used in a variety of ways.
Among these, because of their excellent light resistance properties, leather-like sheet materials using polyester ultra-fine fibers in particular have been used more and more each year in clothing and upholstering and for automobile interiors.
In manufacturing such leather-like sheet materials, a process wherein nonwoven fabric composed of ultra-fine fiber generating type fibers is processed in an organic solvent causing ultra-fine fibers to be appeared is commonly combined with a process wherein, after the nonwoven fabric is impregnated in an organic solvent solution, the fiber sheet material is immersed and cleaned in water, a non-solvent for polyurethane, or in an organic solvent aqueous solution and the polyurethane is caused to undergo wet process hardening.
In the process for appearing ultra-fine fibers, toluene and trichloroethylene or the like are used as organic solvents and a water-miscible organic solvent such as N,N-dimethylformamide used as the polyurethane organic solvent.
However, because organic solvents are highly deleterious to the human body and the environment, when the leather-like sheet materials are manufactured, using a method which does not employ organic solvents is strongly called for.
As a specific resolution step for this requirement, for example, with regard to the process for appearing the ultra-fine fibers, a method wherein ultra-fine fiber processing of fibers is carried out using an alkaline aqueous solution or heating by using an alkaline water solution soluble component or a hot water soluble component for the ultra-fine fiber generating fibers, and with regard to the use of organic solvents for the polyurethane, a method using polyurethane aqueous dispersion in which polyurethane is dispersed in water substituting for a conventional organic solvent type polyurethane are being investigated.
For example, in Patent Document 1, a leather-like sheet manufacturing method is proposed wherein nonwoven fabric composed of fibers for which alkaline extraction is possible is impregnated in a polyurethane aqueous dispersion solution having heat-sensitive gelling characteristics and then extracted in an alkaline aqueous solution. By using a polyurethane aqueous dispersion solution having heat-sensitive gelling characteristics, polyurethane migration when drying after the polyurethane impregnation is suppressed and the texture of the leather-like sheet is made flexible. However, because surfactants are used in this method to impart heat-sensitive gelling characteristics to the polyurethane, surface tackiness is easily generated because of bleeding of the surfactants and as a result, a cleaning process is required after the polyurethane impregnation. Furthermore, because of the presence of the surfactants, fusion of the polyurethane emulsion is easily prevented at film forming time and the film strength of the polyurethane is decreased as is the friction resistance of the sheet material.
Furthermore, in Patent Document 1, appearing the ultra-fine fibers is done with processing in an alkaline aqueous solution after the polyurethane impregnation, but polyurethane generally is easily hydrolyzed in an alkaline aqueous solution. In Patent Document 1, an attempt is made to improve the hydrolysis resistance in an alkaline aqueous solution with polyether or polycarbonate as a polyol, but considering that the urethane bond and the urea bond of the polyurethane are easily hydrolyzed bonds, the separation of the polyurethane during the alkaline aqueous solution processing cannot be controlled and the strength and friction resistance of the sheet material drop remarkably rendering it insufficient for practical use.
In Patent Document 2, a manufacturing method is proposed wherein nonwoven fabric is impregnated after a cross-linking agent is added beforehand to the polyurethane aqueous dispersion solution. By combining with a cross-linking agent, the durability of the polyurethane impregnated nonwoven fabric is improved but when considering its production, when a cross-linking agent is added to and maintained in a polyurethane aqueous dispersion solution, gelling is easily produced because of changes over time and pot life is short.
In Patent Document 3, a manufacturing method is proposed wherein, with the objective of adding suppleness to the polyurethane impregnated nonwoven fabric, polyvinyl alcohol is added to the nonwoven fabric consisting of fibers having a sea component which may be extracted by alkali treatment and then it is impregnated with a polyurethane aqueous dispersion solution. Strengthening the physical properties of the nonwoven fabric during the processing time by adding polyvinyl alcohol, and suppleness of the polyurethane impregnated nonwoven fabric by removal of the paste (removal of the polyvinyl alcohol) after the polyurethane aqueous dispersion solution is imparted polyurethane aqueous dispersion nonwoven fabric are attained. However, it is necessary to go through the two processes of adding the polyvinyl alcohol and removing the paste (removal of the polyvinyl alcohol) and the manufacturing process for the leather-like sheet material becomes extremely long and manufacturing costs increase.
Additionally, it becomes necessary for the leather-like sheet material to have an even higher level of durability.
If it is only intended to increase the friction resistance, high molecular weight elastic bodies may be added in large quantities but if they are added to excess, the texture hardens and suppleness and the quality of the outward appearance, which are characteristics of the original product, are lost. As a result, methods for modifying high molecular weight elastic bodies to obtain highly durable leather-like sheet materials are also being investigated, but so far no products have been obtained with supple texture, excellent outward appearance and durability using polyurethane aqueous dispersion solutions that do not use organic solvents.    Patent Document 1: Japanese patent application Tokkai Publication No. 2001-55670    Patent Document 2: Japanese patent application Tokkai Publication No. 2005-248415    Patent Document 3: Japanese patent application Tokkai Publication No. 2002-317386